OBJECTIVE: This study evaluated autonomic nervous system function in 5 young (6-10 years old) and 5 older (10-15 years) children with recurrent episodes of neurally mediated syncope and a positive tilt-test result, 10 with syncope but a negative test result and 30 age-matched healthy volunteers. METHODS: Time- (SDNN, SDNNi, SDANNi, rMSSD, pNN50) and frequency-domain indices (LF, HF, LF/HF) of heart rate variability (HRV) were measured during a 24-hour Holter recording and for 5 min. segments before and during 90 degrees head-up tilt (tests lasted 30 min. or until syncope occurred). RESULTS: 24-hour HRV-indices were within the normal range for all syncopal patients. Mean RR interval and HRV-indices, except LF/HF, were higher in the older children with a positive test result before they were tilted and during the first 5 min. of head-up tilt. HRV-indices tended to be lower after tilt in the younger children with a positive test result. SDNN and LF in older children with a positive test result and LF/HF in the younger ones increased during the 5 min preceding the syncope. No difference was observed between syncopal children with a negative test result and controls. CONCLUSION: This study confirms that patients with vasovagal syncope have no chronic differences from normal subjects in autonomic nervous system activity. The modulation of the autonomic tone during head-up tilt is abnormal in children who faint during the test, but younger and older patients respond differently to the orthostatic stimulus.